Almost here already? Supposing you fly in to OSL, you could take the train north to Lillehammer (remember the winter Olympics?) and stay yhere for a day. It takes about an hour. In addition to mountains and an older town, there is a huge park filled with old buildings coolected from everywhere in Norway -- "Maihaugen". You will enjoy a full day there and perhaps want to go back the next day.

The train goes on towards Trondheim, but there is a side line towards Åndalsnes you should try. That's the kind of train ride descent where they had to make tunnels to allow for zig-zagging downwards. And not often you see sheer cliff sides falling 1000m I suppose -- "Trollveggen" does.

From Åndalsnes there's a short bus ride to Molde, where you can watch snow covered mountains over the North Sea. The Coastal Steamer passes by, so you can go by ship south towards Bergen, seeing deep fjords on the way or north towards Tondheim, Tromsø or onwards the real far north. It is cheaper to buy tickets in Norway than over the internet, but the cabins tend to get sold out so it may be best to sleep in some of the many villages it visits and take shorter daytrips only.

And there's airports most places, so you will be able to return to OSL whenever you want to. Just rmember that everything is expensive in Norway. Have a nice trip!